86 COMrOUND ORGANS OF PLANTS. 



The fall of the leaf- — There is no subject on which botanists 

 have entertained a greater variety of opinion than on the fall 

 of leaves. The causes which produce their excision from the 

 stems and branches of plants are so exceedingly complicated, 

 thai a much more advanced condition of botanical science seems 

 to be necessary before they will be clearly and accurately 

 understood. It is obvious that leaves are thrown off by plants 

 because they are no longer of any service to them, and the 

 means by which nature effects their separation are truly won- 

 derful, and at the same time instructive. 



The causes which produce the decay and fall of leaves are 

 partly chemical and mechanical. The water which enters the 

 roots of plants as it percolates the soil, dissolves a small portion 

 of earthy matter. This is partly deposited in the woody and 

 fibrous tissues of the stem, but principally in the cellular tissue 

 of the leaves, by the evaporation which is continually taking 

 place at their surface. In this manner the interior walls of the 

 leaf cells become encrusted or thickened by deposits of mineral 

 matter, just as earthy matter accumulates at the bottom of a 

 pot used for culinary purposes, and the leaf is thus rendered 

 finally unfit for the performance of its functions. The mineral 

 matter deposited in the cells is sometimes beautifully crystal- 

 lized, the earths or bases taken up by the roots uniting with the 

 acids formed in the vegetable organs. The most common kinds 

 of crystals are those of the carbonate and oxalate of lime which 

 are of different sizes and forms, rhomboidal, cubical and pris- 

 matic ; but the most prevalent form is the acicular or needle- 

 shaped. It is to this form that the term raphides (raphis a 

 needle) was originally applied by Decandolle, although it is 

 now used indiscriminately in reference to all cellular crystals. 



In the autumnal months, the light becomes less powerful, the 



